1. Field of the Invention
The present invention relates to a ceramic substrate and to a method for producing the same and, more particularly, to a ceramic substrate widely used as a multilayer interconnection substrate for being loaded with electronic parts and to a method for producing the same.
Recently, as for a multilayer interconnection substrate loaded with a highly integrated LSI device and various sorts of electronic parts, in order to miniaturize electronic equipment, improve the reliability and so on, more ceramics have been used as a substrate material. And as for such ceramic substrate, there are severe requirements in various characteristics such as sintering temperature, specific inductive capacity, coefficient of thermal expansion, transverse strength and water resistance.
2. Description of the Relevant Art
Due to having the advantage such as a high strength, the proportion of alumina in the ceramic for a substrate material is large. On the other hand, however, alumina has problems of having a large specific inductive capacity, which causes delays of transmission signals, and having a coefficient of thermal expansion much higher than silicon, which makes it difficult to ensure the reliability in packaging parts. And alumina has the other problems of having a high sintering temperature of about 1550.degree., which requires the use of W or Mo with a high melting point and a high electric resistivity as a material for an interconnection pattern formed inside a sintered body (hereinafter referred to as an interconnection inside layer), and having a higher electrical resistance by making the interconnection pattern refined.
Therefore, in order to deal with these problems, the study and development of a low temperature sintered ceramic which has both a smaller specific inductive capacity and a coefficient of thermal expansion closer to silicon, and which can be sintered even with the use of a metallic material having a low melting point and a low resistivity such as Cu, Ag or Ag--Pd as a material for forming an interconnection inside layer (hereinafter referred to as a conductor inside layer), has been promoted.
Generally, a low temperature sintered ceramic substrate is produced by mixing a glass material with a crystal material called a filler and sintering the same. However, the number of combinations of glass materials with crystal materials is extremely large and every combination of both of them makes a different synergistic effect in sintering, resulting in obtaining a ceramic substrate having different characteristics (specific inductive capacity, coefficient of thermal expansion, sintering temperature, transverse strength, etc.). Therefore, it is difficult to find the best combination and produce a ceramic substrate having such composition and structure so as enable constant and stable characteristics to usually appear.
In such a situation, as a low temperature sintered ceramic substrate, whose strength can be large without impairing the characteristics of a low specific inductive capacity and a coefficient of thermal expansion close to silicon, and which can cope with higher speed transmission of signals and larger size elements such as a LSI device loaded with a substrate, a cordierire (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2)-base crystallized glass as disclosed in Japanese Patent Laid-Open Publication No.225338/90 and a composite of a cordierire crystallized glass with a ceramic filler as disclosed in Japanese Patent Laid-Open Publication No.225339/90 and No.225340/90 have been noticed.
However, since the cordierire-base crystallized glass disclosed in the Japanese Patent Laid-Open Publication No.225338/90 has a high softening point land a high viscosity at a high temperature, it is difficult to make the substrate dense.And since the composite of a cordierire crystallized glass with a ceramic filler disclosed in the Japanese Patent Laid-Open Publication No.225339/90 and No.225340/90 is made with adding a ceramic filler to a cordierire having a high softening point, it is also difficult to make the ceramic substrate dense by sintering at a low temperature.
Hence, the ceramic substrate obtained by sintering so as to grow a cordierire-base crystallized glass at a temperature of less than 950.degree. C. required to use Ag as a material for an interconnection inside layer, or less than 1000.degree. C. required to inside layers, does not have a sufficiently small porosity, a sufficient transverse strength, a sufficient moisture resistance and so on, and the oxidation and the migration of its conductor inside layers easily occurs, making the substrate less reliable.
And hitherto, in the above method for growing a crystal from a glass, it is usual to add a nuclear formation material (an aggregate) to a glass component in order to make it easy for a crystal nucleus to be formed in the glass. In this case, however, the heat treatment for nuclear formation such as maintaining a crystal formation temperature for a definite time is usually required, resulting in making the sintering time of the substrate longer and the productivity poorer.